This invention relates to magnetic resonance imaging apparatus and more specifically to gradient amplifier systems for use in such apparatus.
Magnetic resonance imaging ("MRI") has developed as an important tool in diagnostic medicine. In MRI, as is understood by those skilled in the art, a body being imaged is held within a uniform magnetic field oriented along a z axis of a Cartesian coordinate system.
The spins of the nuclei of the body are excited into precession about the z axis by means of a radio frequency (RF) pulse and the decaying precession of the spins produces an NMR signal. The amplitude of the NMR signal is dependant, among other factors, on the number of precessing nuclei per volume within the imaged body termed the "spin density".
Magnetic gradient fields G.sub.x, G.sub.y, and G.sub.z are applied along the x, y and z axes, by means of gradient coils driven by a gradient amplifier system, so as to impress position information onto the NMR signals through phase and frequency encoding. A set of NMR signals may then be "reconstructed" to produce an image. Each set of NMR signals is comprised of many "views", a view being defined as one or more NMR signal acquisitions made under the same x and y gradients fields.
Referring to FIG. 1, a typical "spin echo" pulse sequence for acquiring data under the spin warp MRI technique includes: 1) a z-axis gradient G.sub.z activated during a first 90.degree. RF pulse to select the image slice in the z axis, 2) a y-axis gradient field G.sub.y to phase encode the precessing nuclear spins in the y direction, and 3) an x-axis gradient G.sub.x activated during the acquisition of the NMR signal to frequency encode the precessing nuclear spins in the x direction. Two such NMR acquisitions, S.sub.1 and S.sub.1 ', the latter inverted and summed with the first, comprise the NMR signal of a single view "A" under this sequence. Note that the y gradient field G.sub.y changes between view "A" and subsequent view "B". This pulse sequence is described in detail in U.S. Pat. No. 4,443,760, entitled: "Use of Phase Alternated RF Pulses to Eliminate Effects of Spurious Free Induction Decay Caused by Imperfect 180 Degree RF Pulses in NMR Imaging", and issued Apr. 17, 1984 and assigned to the same assignee as the present invention.
It will be apparent from the above example that the energy required to develop the gradient fields ("gradient") for a particular scan will vary significantly between gradient axes. In general, the power demanded by the gradients is unequal, with the dominant axis with regard to power consumption varying depending on the orientation of the slice sequence and the particular imaging technique used. It is not unusual for one gradient to require two to three times more power than the other gradients and in certain imaging techniques one gradient may require over five times as much power as the other gradients. The power required to generate a gradient field depends both on the peak amplitude of the gradient field, which is proportional to the current in the gradient coil, and on the rate of rise of magnetic field required by that gradient, which is proportional to the voltage applied across the coil.
Each gradient amplifier block is sized to handle the peak load it will experience during the widest feasible range of imaging techniques. This preserves a flexibility in the types of imaging sequences that may be performed on the MRI equipment. However, as a result of the variation in power use among the gradients, it is inevitable that in most scanning sequences one or more gradient amplifiers will be running at substantially less than full capacity.
Each gradient amplifier may contain eighty power transistors as required to produce adequate power to drive the gradient coils. For reasons of economy, each power transistor is operated near the limits of its power ratings. The resultant "semiconductor wear", caused by repeated thermal stress, significantly affects the life of the amplifier. When the amplifiers are connected in series to produce a gradient voltage, they carry equal current and hence experience equal semiconductor wear. Unnecessary amplifiers therefore only increase the total semiconductor wear.